In the oil and gas industries, petrochemicals and hydrocarbon gases are extracted from deep in the earth through pressure bearing tubulars or “tubing”. The tubing forms a conduit from the rock where the petrochemicals reside to the surface where it is terminated at the Wellhead or Christmas Tree. The wellhead is equipped with a number of valves to control and contain the pressure which is present in the tubing.
The oil or gas flows from source rock which may exist in a layer of just a few feet to many hundreds of feet. The quality and productivity of the rock may vary over distance and water or other undesirable elements may exist at certain points. Usually it is best practice to produce over the entire oil bearing interval and for any water to be produced along with the oil. Towards the latter stages of a well's life, the water production will generally increase at the expense of oil production. Production optimisation will depend on minimising the water production which will maximise the oil production.
Production may also be lost to “thief” zones. Thief zones are areas of rock penetrated by the wellbore which have less pressure than others. Crossflow can occur from a good high pressure zone to a poor low pressure zone (see FIG. 1). Obviously, this is inefficient. Production optimisation will depend on isolating the thief zone until such time as the good high pressure zone has depleted to the extent that the pressure is the same or lower than the thief zone. Once the isolation has been removed, both zones may be allowed to flow to surface.
The production may initially be optimised by “shutting off” thief zones or water producing zones. Firstly, these zones must be identified and targeted. Instruments lowered into the wellbore on a wireline cable allow pressure, temperature, flow measurement and flow composition readings to be taken. Following analysis, a second intervention into the well may be conducted to mechanically close off the undesirable zone(s). A variety of equipment is available for this but most will dictate permanently closing off a part of the wellbore which action may be undesirable in later years.
A technology whereby the zones of a well may be individually opened or closed to help optimise the production from that well is called “smart well” technology. Differing zones are mechanically separated and isolated by packer assemblies (see FIG. 2). Flow from the zones is received through a valve which may allow on/off or incremental flow. Most of these valves feature a sleeve which uncovers flow ports in the outside diameter of the tool. Many of these valves may be installed in a well with surface control being provided by means of electric cables, hydraulic control lines or other means. Most smart well systems require a physical link from the bottom of the well or the valve apparatus to surface in order to provide hydraulic contact, electrical contact or both. Not only is this expensive, it becomes a source of unreliability. Failure of one part of this type of system may compromise all of the system. Obviously, the complexity (and unreliability) of the installation increases proportionally with the number of valves and the increase in control lines and/or electric lines, splices and connections.
Equipment which uses this type of physical link must be installed when the well is new. It is not capable of retrofitting into an existing well.
The ability to repeatedly open and close various zones from surface allows true optimisation without the need to intervene in the well for data collection or for installation of shut off equipment. Also, isolated zones may easily and quickly be re-opened for evaluation and potential production later in the life of the well or simply just for re-evaluation purposes.
Many wells are not suited to intervention techniques due to the great cost associated with these operations. These may be subsea wells where no facilities exist to support the intervention, high pressure wells where safety is a prime consideration or remote wells where also, no facilities exist.
Recent innovations in the electro mechanical and acoustic fields have sought to mitigate the disadvantages of the physical link to surface and associated unreliability. These devices may offer a greater degree of flexibility and possibly higher reliability in the future. These technologies are as yet unproven and may have undesirable issues of their own such as limited range, high power consumption and lack of proven operation.
Accordingly, the present applicants sought to provide an alternative means of smart well operation with no boundaries of range and great service life due to low power consumption. This resulted in the apparatus and method which was the subject of International Application No. PCT/GB2005/01793.
Whilst the invention which was the subject of the above identified Application addressed a significant problem with the prior methods and apparatus, discussed in detail above, there is a desire to improve upon the disclosed method and apparatus. In particular, the applicants have recognised that the elevated temperatures which are experienced downhole can impact upon performance of the apparatus.
In more detail, the disclosed method and apparatus involves the monitoring of a characteristic pressure profile of a well during a certain time span, and selective triggering of an actuator to perform any required operation of the well. Such might involve the opening or closing of a valve downhole, to control fluid flow. Triggering of the actuator occurs when the same pressure profile is monitored twice within the time span, which may be a certain hour of the day on separate days. The apparatus employs a downhole clock, which controls operation of a pressure monitoring device. The pressure monitoring device is arranged to monitor the pressure profile during the desired time span according to the time outputted by the clock. This requires correlation between the output of the downhole clock and a real-time clock at surface, so that the control signal is sent at the appropriate time.
The elevated temperatures experienced downhole can, however, cause ‘drift’ between the time measured by the clock downhole and real time. For example, the clock may typically experience temperatures downhole of around 150° C. In a modern electronic clock employing a timing crystal, this can lead to a drift of around 1 minute per day relative to real time. The clock thus effectively runs around one minute faster per day when exposed to such temperatures. These discrepancies can build up over a period of days, months and years, and could potentially result in a control signal being sent at a time when it is expected that the device would be looking for the signal, but in which the device is actually inoperative. A desired actuation operation would not therefore by carried out. The invention which is the subject of the present application seeks to address this issue.